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Scope:

For decades, the aluminum industry has been investigating options for
treating, disposing, and
using bauxite residue (or red mud), a byproduct of the Bayer process
to extract aluminum oxide from
bauxite ore. This report examines the issues surrounding bauxite
residue, discusses different
approaches for treatment/disposal/utilization, and identifies research
and development needs for
some of the most promising options.

Alumina Production and Bauxite Residue

The starting material for electrolytic smelting of aluminum is pure,
anhydrous aluminum oxide
(Al2O3), also known as alumina. The principal ore from which aluminum
is extracted is called
bauxite. The principal ingredients contained in the bauxite are
hydrated aluminum oxide, iron oxide
(which gives it its reddish-brown color), silicates (e.g., clay,
quartz), and titanium dioxide.

The Bayer process is used to produce alumina from bauxite. Because of
variability in ores, each
alumina plant is almost tailored to suit a particular bauxite. In the
Bayer process, the bauxite is
crushed and ground, then mixed with a solution of caustic soda and
pumped into large autoclaves.
There, under pressure and at a temperature of 110°C to 270°C,
the alumina contained in the
ore is dissolved to form sodium aluminate. The silica in the bauxite
reacts and precipitates from
solution as sodium-aluminum-silicate.

The remaining residues consist partly of minerals that do not dissolve
during the caustic treatment
of the bauxite (e.g., iron and titanium oxide), calcium carbonate and
calcium aluminates from lime
addition, and also of what is known as the dedication product (DSP),
which contains not only silica
but considerable quantities of unrecoverable alumina and soda. Some
bauxite residues contain as
much as 24% TiO2, 43% Al2O3, and 54% Fe2O3, depending on the
mineralogical make-up of the bauxite.
The residue also contains trace amounts of the metals niobium,
gallium, zirconium, thorium,
scandium, and vanadium. The residue settles out of solution and is
separated from the sodium
aluminate solution, washed to recover the caustic soda, and pumped to
disposal areas.

The worldwide alumina industry produces over 70 million dry metric
tons of bauxite residue
annually. Australia is the largest alumina refiner in the world,
processing around 30% of the total
alumina (see text box next page).

The production of one ton of aluminum metal yields 1.5 to 4 tons of
bauxite residue (on a dry
basis) to be disposed. The worldwide average of residue in disposal
sites is 50% solids in the
residue.

Overview of Bauxite Residue Treatment

Bauxite residue is removed during the Bayer process by gravity
settling or filtering the solids
from the sodium aluminate liquor. Storage methods, which involve the
dewatering of the residue, are
tailored to suit local situations and legislation. Methods that do not
preclude the future
exploitation of the residue can be considered under the headings
stacking and nonstacking.

Stacking calls for residue that is dense enough to require (if it is
to be conveyed) vacuum,
pressure, or high-pressure filtration to ensure the desired degree of
dewatering. Alternatively, a
common stacking method consists of spreading a thin layer of residue
over a broad area and then
extending the next layer only when the first is completely dry.

Nonstacking methods provide for the discharge of bauxite residue into
lagoons. Such lagoons may not
be naturally impervious but can be made so by means of plastic
sheeting or clay to prevent seepage
of the caustic liquid into the subsoil. The mud that settles on the
bottom can be drained and
stored. A layer of water is kept over the surface of the mud, or the
lake is sprayed periodically
with water to keep the dust hazard to a minimum. Eventually, lagoons
can be covered with a layer of
topsoil when they are closed.